I like all the advise given here. Some seems better than others. Getting some type of admin involved never seemed to work for me, I guess I couldn't plead my case as well as the NT bullies.
My Mother taught me a technique, however, that has stood me in good stead and never failed. She said: "Get them alone, one at a time."
For me, in the '40s and '50s, this worked well because, then, according to most people and especially admin types, physical bullying was just a fact of life, like bad weather and "It always takes two to make a fight" (idiots). There was no help from school authorities at all.
But these days perhaps you don't have to physically "adjust" the situation. Some bully types are only followers...by themselves they may find they think differently and desist. I've also found bullies are often cowards looking for someone more miserable to take their frustrations out on. Never give in.